1. Field of the Invention
The present invention relates to a receiving circuit and a timepiece having the receiving circuit.
2. Description of the Related Art
In some country, a long-wave standard radio wave carrying a time code is transmitted from a transmitting station. Radio-controlled clocks are in practical use, which receive the standard wave to detect the time code and accurately adjusts the time of day based on the detected time code. In Japan, there are two transmitting stations. The transmitting station transmits the standard wave of a carrier frequency 40 kHz and the standard wave of a carrier frequency 60 kHz respectively. The time code is frame data of a period 60 sec.
In a receiving circuit for receiving the standard wave, for instance, in a superheterodyne receiving circuit, the standard wave of the carrier frequency 40 kHz and a local oscillation signal are mixed together to create an intermediate frequency signal of an intermediate frequency (for example, 30 Hz), and the created intermediate frequency signal is supplied to a filter to remove the intermediate frequency component included therein.
FIG. 15 is a view showing a conventional frequency converter circuit and detecting circuit. The standard wave signal of the carrier frequency 40 kHz is amplified by an amplifier 200, and further applied to a mixer 202. The mixer 202 mixes the amplified standard wave signal with a local oscillation signal (for instance, 39.97 kHz) of OSC (local oscillator) 201 to produce an intermediate frequency signal of a frequency 90 Hz. The produced intermediate frequency signal is supplied to BPF (band pass filter) 203, which allows only frequency components within a frequency range close to 90 Hz to pass through. The signal passing through the filter is supplied to a detector 205 through an amplifier 204. The detected signal is supplied to LPF (low pass filter) 206. At LPF, an intermediate frequency component is completely removed from the detected signal, whereby only a desired signal is obtained.
U.S.P. 2005-0260957 A1 discloses a technique for receiving the long-wave standard radio wave, in which a frequency multiplier switches between local oscillation signals to be supplied to a frequency converter, whereby an intermediate frequency signal is created both from the radio wave signals of 40 kHz and 60 kHz. An oscillator is also disclosed, which has an oscillation signal of a frequency in the range between two frequencies.
As described with reference to the receiving circuit shown in FIG. 15, when the intermediate frequency is low in the range of about 90 Hz, it is necessary for adequately removing the intermediate frequency component to reduce the cut off frequency of LPF and/or to use a high order filter. It is necessary for sufficiently integrating the intermediate frequency signal of 90 Hz to increase the time constant of LPF. For instance, if the cut off frequency f0 of LPF is set to 5 Hz to allow a signal of 5 Hz pass through, it is found that the signal component of 90 Hz attenuates only to the extent of 25 dB, leaving a ripple component.
Reducing the cut off frequency of LPF brings a disadvantage that a modulated signal is distorted. If the high order filter is used, another problem is invited that the circuit is made complex and the circuit scale becomes large.
The present invention has an object to provide a receiving circuit including a frequency converting/detecting circuit and to provide a time piece having the receiving circuit, wherein the frequency converting/detecting circuit has a simple circuit configuration and is capable of receiving a modulated signal in high accuracy.